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.\" RCSid $Id: genBSDF.1,v 1.5 2011/05/06 23:58:09 greg Exp $
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.TH GENBSDF 1 9/3/2010 RADIANCE
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.SH NAME
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genBSDF - generate BSDF description from Radiance or MGF input
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.SH SYNOPSIS
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.B genBSDF
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[
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.B "\-c Nsamp"
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][
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.B "\-n Nproc"
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][
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.B "\-r 'rtcontrib opts...'"
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][
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.B "\-t{3|4} Nlog2"
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][
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.B "{+|-}forward"
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][
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.B "{+|-}backward"
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][
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.B "{+|-}mgf"
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][
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.B "{+|-}geom
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][
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.B "\-dim Xmin Xmax Ymin Ymax Zmin Zmax"
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]
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[
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.B "geom .."
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]
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.SH DESCRIPTION
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.I GenBSDF
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computes a bidirectional scattering distribution function from
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a Radiance or MGF scene description given on the input.
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The program assumes the input is in Radiance format unless the
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.I \+mgf
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option is specified.
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The output conforms to the LBNL Window 6 XML standard for BSDF data,
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and will include an MGF representation of the input geometry if the
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.I \+geom
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option is given.
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(This is the default, and may be switched off using
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.I \-geom.)\0
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.PP
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Normally,
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.I genBSDF
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computes components needed by a backwards ray-tracing process,
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.I \+backward.
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If both forward and backward (front and back) distributions are needed, the
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.I \+forward
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option may be given.
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To turn off backward components, use the
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.I \-backward
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option.
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Computing both components takes about twice as long as one component.
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.PP
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The geometry must fit a rectangular profile, whose width is along the X-axis,
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height is in the Y-axis, and depth is in the Z-axis.
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The positive Z-axis points into the room, and the input geometry should
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not extend into the room.
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(I.e., it should not contain any positive Z values, since the putative
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emitting surface is assumed to lie at Z=0.)\0
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The entire window system should be modeled, including sills and
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edge geometry anticipated in the final installation, otherwise
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accuracy will be impaired.
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Similarly, materials in the description should be carefully measured.
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.PP
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Normally, the input geometry will be positioned according to its actual
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bounding box, but this may be overridden with the
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.I \-dim
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option.
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Use this in cases where the fenestration system is designed to fit a
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smaller (or larger) opening or is offset somehow.
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.PP
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The variance in the results may be reduced by increasing the number of
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samples per incident direction using the
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.I \-c
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option.
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This value defaults to 1000 samples distributed over the incoming plane
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for each of the 145 Klems hemisphere directions.
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.PP
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In some cases, the processing time may be reduced by the
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.I \-n
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option, which specifies the number of simultaneous
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.I rtrace(1)
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processes to run in
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.I rtcontrib(1).
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The
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.I \-r
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option may be used to specify a set of quoted arguments to be
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included on the
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.I rtcontrib
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command line.
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.PP
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The
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.I \-t4
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mode computes a non-uniform BSDF represented as a rank 4 tensor tree,
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suitable for use in the Radiance rendering tools.
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The parameter given to this option is the log to the base 2 of the
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sampling resolution in each dimension, and must be an integer.
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The
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.I \-c
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setting should be adjusted so that an appropriate number of samples
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lands in each region.
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A
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.I \-t4
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parameter of 5 corresponds to 32x32 or 1024 output regions, so a
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.I \-c
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setting of 102400 would provide 100 samples per region on average.
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Increasing the resolution to 6 corresponds to 64x64 or 4096
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regions, so the
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.I \-c
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setting would need to be increased by a factor of 4 to provide
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the same accuracy in each region.
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.PP
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The
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.I \-t3
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mode is similar to
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.I \-t4
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but computes a rank 3 tensor tree rather than rank 4.
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This provides a much faster computation, but only works
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in special circumstances.
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Specifically, do NOT use this option if the system is not in fact isotropic.
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I.e., only use
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.I \-t3
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when you are certain that the system has a high degree of radial symmetry.
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Again, the parameter to this option sets the maximum resolution as
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a power of 2 in each dimension, but in this case there is one less
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dimension being sampled.
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.SH EXAMPLE
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To create a BSDF description including geometry from a set of venetian blinds:
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.IP "" .2i
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genblinds blind_white blind1 .07 3 1.5 30 40 | xform -rz -90 -rx 90 > blind1.rad
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.br
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genBSDF -r @rtc.opt blind_white.mat glazing.rad blind1.rad > blind1.xml
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.PP
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To create a non-uniform, anisotropic BSDF distribution with a maximum
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resolution of 128x128 from the same description:
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.IP "" .2i
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genBSDF -r @rtc.opt -t4 7 -c 160000 blind_white.mat glazing.rad blind1.rad > blind12.xml
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.SH NOTES
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The variable resolution (tensor tree) BSDF representation is not supported
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by all software and applicatons, and should be used with caution.
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It provides practical, high-resolution data for use in the
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Radiance rendering programs, but does not work in the matrix formulation
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of the daylight coefficient method for example.
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Also, third party tools generally expect or require a fixed number of sample
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directions using the Klems directions or similar.
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.SH AUTHOR
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Greg Ward
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.SH "SEE ALSO"
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dctimestep(1), genklemsamp(1), genskyvec(1), mkillum(1), rtcontrib(1), rtrace(1)
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